Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
2002-04-03
2004-04-27
Lebentritt, Michael S. (Department: 2824)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S066000, C365S171000
Reexamination Certificate
active
06728132
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of nonvolatile memory devices for use as computer storage, and in particular to nonvolatile memory arrays that use magnetic memory elements as the individual data bits.
BACKGROUND OF THE INVENTION
Integrated circuit designers have always sought the ideal semiconductor memory: a device that can be randomly accessed, written or read very quickly, is nonvolatile but indefinitely alterable, and consumes little power. Magnetic Random Access Memory (MRAM) technology has been increasingly viewed as offering many of these advantages.
An MRAM device typically includes an array of magnetic memory elements
11
located at the intersections of row line
13
and column line
15
conductors as illustrated in
FIG. 1. A
simple magnetic memory element has a structure which is shown in more detail in FIG.
2
. The magnetic memory element includes ferromagnetic layers
17
and
19
separated by a nonmagnetic layer
21
. The magnetization in one ferromagnetic layer
17
, typically referred to as the pinned layer, is fixed in one direction. The magnetization of the other ferromagnetic layer
19
, often referred to as the sense layer, is not pinned, so its magnetization is free to switch between “parallel” and “anti-parallel” states relative to the magnetization of the pinned layer. The sense layer may also be referred to as the “free” layer or storage layer, thus it should be understood that when the term “sense layer” is used, the meaning of this term should not be limited to this terminology but rather the function the layer performs.
The logical value or bit stored in an MRAM memory element is associated with a resistance value, and the resistance of the memory element is determined by the relative orientation of the sense layer magnetization with respect to the pinned layer magnetization orientation. A parallel orientation of the magnetization of the sense layer with respect to the pinned layer magnetization results in a low resistance. Conversely, in response to the anti-parallel orientation, the magnetic memory element will show a high resistance. Referring to
FIG. 2
, the manner in which the resistance of the memory element is read is dependent on the type of material used to form the nonmagnetic spacer layer
21
separating the pinned layer
17
from the sense layer
19
. If the nonmagnetic spacer layer
21
is made from a conducting material, such as copper, then the resistance value of the memory element can be sensed via the giant magnetoresistance effect, which usually involves running a current parallel to the long axis
20
of the memory element. If the nonmagnetic spacer layer is composed of an insulating material, such as alumina, then the resistance value can be sensed using the tunneling magnetoresistance effect, and this is accomplished by running a current perpendicular to the plane of the nonmagnetic spacer layer
21
from the sense layer
19
to the pinned layer
17
.
A logical “0” or “1” is usually written into the magnetic memory element by applying external magnetic fields (via an electrical current) that rotate the magnetization direction of the sense layer. Typically an MRAM memory element is designed so that the magnetization of the sense layer and the pinned layer aligns along an axis known as the easy axis
27
. External magnetic fields are applied to flip the orientation of the sense layer along its easy axis to either the parallel or anti-parallel orientation with respect to the orientation of the magnetization of the pinned layer, depending on the logic state to be stored.
MRAM devices typically include an orthogonal array of row and column lines (electrical conductors) that are used to apply external magnetic fields to the magnetic memory elements during writing and may also be used to sense the resistance of a memory element during reading. Additional write and read conductors may also be present in the array. In the two conductor level implementation shown in
FIG. 1
, the magnetic memory elements are located at the intersections of the row
13
and column
15
lines.
Referring to
FIG. 1
, the magnetic field that is generated by running a current through the column line
15
is referred to herein as the easy-axis write field. The easy-axis write field is collinear with the easy axis
23
of the MRAM bit
11
(
FIG. 2
a
). The magnetic field that is generated when a current is run through the row conductive line
13
is referred to as the hard-axis write field. The hard-axis write field generated by the row conductive line
13
runs parallel to the hard axis
25
of the MRAM bit
11
.
A memory element is selected for writing when it is exposed to both a hard-axis and an easy-axis write field. Each write field, by itself and when generated with only one of the two conductive lines, is therefore commonly referred to as a half-select field because a single field by itself should not be of sufficient magnitude to switch the magnetization orientation of the sense layer of a memory element. In practice, however, the hard-axis write field is often referred to as the half-select field, while the easy-axis write field is often referred to as the switching field. These two fields are used to perform write operations on a specific memory element when applied in conjunction with each other by passing current through conductors
13
,
15
(
FIG. 1
) intersecting at the selected element. The bit stored at the selected memory element being accessed for a read or write operation is referred to herein as a “selected bit”.
This method for selecting a bit for writing is not ideal. During a write operation, the unselected memory elements coupled to the particular column line
15
are exposed to the easy-axis write field. Similarly, the unselected memory elements
11
coupled to the particular row line
13
are exposed to the hard-axis write field. It is thus important to limit stray magnetic fields in the array of MRAM memory elements to a value that cannot cause half-selected bits to be written. Some sources of stray fields include fields from neighboring write conductors, stray fields emanating from the ferromagnetic layers of neighboring memory elements, and fields generated by sources external to the MRAM device. These stray fields may also inhibit a selected memory element from being written, if the combined value of the stray, hard-axis, and easy-axis fields is too small for a bit to be written. Another source of non-ideal behavior that manifests itself in the write current required to write a memory element results from the difficulty in making an array of MRAM memory elements that respond identically to the applied write fields. Some sources of this effect include variations in element-to-element geometry, variations in element-to-element magnetic properties, and thermally activated magnetization fluctuations. Therefore, the particular value of the hard and easy-axis write fields, and thus the row and column line write currents, is a compromise such that selected memory elements are selected with enough margin that they are always written and unselected memory elements are never exposed to a field large enough that they are unintentionally written.
Thermal effects, such as superparamagnetism or thermally activated magnetization reversal, and the effect of stray fields emanating from neighboring bits may cause problems in MRAM devices. Either of these of these mechanisms can result in unpredictable write and read behavior. Using a conventional single-layer sense layer, these effects will be extremely difficult to overcome as the bit density of the MRAM device is increased.
Thermal fluctuation in a seemingly unfluctuating macroscopic observable quantity, such as the magnetization of a ferromagnetic material, is an abstract concept. The orientation and magnitude of the magnetization of a ferromagnetic material are in actuality statistical quantities. In any material, fluctuations in thermal energy are continually occurring on a microscopic scale, where the magnitude of the thermal fluctuations is determin
Lebentritt Michael S.
Micro)n Technology, Inc.
Nguyen Tuan T.
LandOfFree
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